1. Field of the Invention
The present invention relates generally to the field of temperature measurement using optical techniques and, more particularly, to a temperature sensor employing a photoelastic substrate having a stress induced optical waveguide with temperature dependent index of refraction.
2. Description of the Prior Art
Various measuring structures are known for temperature sensing by measuring the intensity of a light beam. Usually associated with a fiber optical transmission line, these measuring devices have included birefringent temperature sensors which exploit temperature dependent birefringing effects of various crystals, the temperature dependent phosphorescence decay time of phosphors, and interference and absorption effects in crystals employing optical waveguides with temperature dependent refractive indices. Such optical waveguides have utilized an optically transparent crystal such as gallium arsenide (GaAS), lithium tantalate (LiTaO.sub.3) or lithium niobate (LiNbO.sub.3) and form a channel for guiding light wave energy characterized by a change in the refractive index of the material. The refractive index change leads to the formation of an optical waveguide in a region wherein the refractive index is increased relative to the surrounding area, or decreased relative to the surrounding area. Light travelling in a medium having a transverse variation in refractive index is reflected towards regions having the larger refractive index, as is well known in the art.
Such waveguides have heretofore been formed, for example, by diffusing a transition metal such as titanium (Ti) into a LiNbO.sub.3 crystal substrate to form a guiding layer of increased refractive index, as by evaporating a thin layer of the metal on the surface of the crystal and then heating the crystal to a suitable temperature for diffusion into the substrate.
In photoelastic Effect Optical Waveguides, U.S. Pat. No. 4,561,718, filed Aug. 22, 1983 and issued Dec. 31, 1985 to the present inventor and assigned to the assignee of the present invention, photoelastic guides were disclosed, using evaporated metal or insulating dielectric stripes on LiTaO.sub.3 and LiNbO.sub.3 substrates. These guides, which are capable of multi-mode propagation, were observed to offer significant advantages over the previously used single-mode Ti-diffused guides in LiNbO.sub.3, as reported by the present inventor in Photo-Elastic Waveguides in LiTaO.sub.3 and LiNbO.sub.3, Appl. Opt. 19, 3423 (1980). In devices using the photoelastic effect, the waveguide is caused, at least in part, by the effect of changing the refractive index by the stress field in the semiconductor material surrounding a deposited stripe, or in a window formed between a plurality of such stripes. This stress field often results from the state of compression or tension induced by the deposited film due to the differing thermal expansion coefficients of the substrate and the deposited film after cooling from the elevated temperature required for deposition. Using the photoelastic effect, it is possible with the correct pattern of an evaporated film to produce regions of permanently increased and decreased refractive index that will guide light without application of bias voltages.
Interferometer-type integrated optics structures making use of the index change of the guide and coupling region and the change in guide length with temperature were described by L. M. Johnson and G. W. Pratt in Integrated Optic Temperature Sensor, Digest, Third International Conference on Integrated Optics and Optical Fiber Communication, Apr. 27-29, 1981, pp 130-131. However, this system and similar interferometer devices using the technology of single mode transmission through Ti-diffused guides in LiNbO.sub.3 has relatively low sensitivity and restricted dynamic range. The modulated light output goes through periodic maxima and minima, requiring additional circuit complexity for counting the cyclical variations or otherwise compensating for such ambiguities over an extended temperature range.
Another arrangement based on semiconductor absorption with temperature offers a continuous relation of output voltage vs. temperature over a limited dynamic range, but is subject to errors induced by variations in power at the light source, thereby requiring incorporation of a second light source to provide a reference. This structure was described by K. Kyuma, et al, Fiber-Optic Measurement Instrument for Temperature, Digest, Conference on Lasers and Electro-optics, June 11, 1981, pp. 102-103.
A further optical temperature sensor using the principle of varying light intensity by reflection from semiconductor dielectric interfaces with a temperature dependent index of refraction was disclosed by H. Kroger and R. A. Soref in Refractive Index Sensor, U.S. Pat. No. 4,437,761, filed Mar. 27, 1981, and assigned to the present assignee. However, this sensor is limited by choice of the reflecting dielectric parameters to a compromise between dynamic range and temperature sensitivity.
The present invention is comprised of an integrated optic sensor with increased sensitivity and dynamic range over the prior art, providing a direct conversion of temperature deviation to intensity modulation of the light signal which is substantially insensitive to undesirable environmental perturbations. The sensors described use photoelastic waveguides and either single mode or multimode technology.